Concentration of minerals



Patented Sept. 28, 1954 UNITED STATES PATENT OFFICE 2,690,261 CONCENTRATION or MINERALS Ernest J. Maust, Packanack Lake, N. J., assignor to Western Machinery Company, New York, N. Y., a corporation of Utah Application -March 2, 1951, Serial No. 213,630

2 Claims.

This invention relates to the concentration of minerals by the heavy media separation process, and has for its object the provision of certain improvements in that process, and more especially in the recovery of heavy medium carried out of the heavy medium circuit along with the float and sink fractions of theseparation. .A further object of the invention is the provision of certain improvements for the disposal of slimes produced in the normal operation of the heavy media separation process.

The heavy media separation process, sometimes called the sink and float process, consists in introducing a mixture of solid particles of different specific gravities to be separated, such for 7 example as crushed coal, ore or other mineral agglomerate, into a heavy (dense) liquid or medium of. a specific gravity between'the specific gravities of the solid particles (e. g. valuable and waste constitutents of the crushed mineral) to be separated. Solid particles having a specific gravity less than that of the heavy medium float to the surface while solid particles having a specific gravity greater than that of the heavy medium sink in the medium, and the resulting'float and sink fractions of the separation are appropriately removed from the separatory apparatus, such as a drum, cone, trough or the like. Usually the heavy medium is an aqueous suspension of a finely divided solid, such, for example, as finely ground galena, barytes, silica, hematite, magnetite, ferrosilicon and mixtures of one or more of these with other minerals. This finely divided solid constituent of the heavy medium is herein referred to as the heavy mediums solid constituent. I

The maintenance of a circulating heavy medium circuit through the separatory apparatus of substantially constant specific gravity and free from contaminating slimes, and the recovery of heavy medium carried away from the circuit along with the float and sink fractions are essential to the successful operation of the heavy media separation process. The present invention is particularly directed to such recovery of heavy medium and to the, removal of slimes produced in the normal functioning of the process, and is applicablewhere the mixed mineral particles of difierent specific gravities are subjected to wet screening preparatory to the separation step and the float and sink fractions of the separation step are respectively drained and washed on a sequence of screens. For carrying out the invention, the screen cloth or perforated plate of the wet preparatory screen has apertures or openings sub- Stantially larger than the apertures or openings of the screen cloth or perforated plate of the draining and washing screens over which the float and sink fractions pass after removal from the separatory apparatus. Conversely stated, the aperture size or mesh of the draining and washing screens is finer or smaller than the aperture size or mesh of the'wet preparatory screen. This difierence in size of screen aperture prevents contamination of the heavy medium with naturallyoccurring fine mineral particles which pass the wet preparatory screen and enter the separatory apparatus with the feed of mineral particles. Since such naturally-occurring fine mineral particles cannot pass the smaller openings of the draining and washing screens, the heavy medium underflow of the draining screens and the underflow of mixed heavy medium and wash water from the washing screens are each substantially free of such contaminating naturally occurring fine mineral particles.

The improvements of the invention involve the separation of the heavy mediums solid constituent from the slime'particles mixed therewith in the underfiow of the aforementioned washing screens, recovery of the separated heavy mediums solid constituent, and disposal of the slime particles. It is inherent in the heavy media separation process that the specific gravity of the heavy mediums solid constituent is greater than the specific gravity of those particles of the feed that float. Slime produced from these light weight (low specific gravity) particles when included in the heavy medium dilute the medium, and thereby reduce its specific gravity and increase its viscosity. Slime produced from the heavy weight (high specific gravity) particles of the feed, if of the same or higher specific gravity than the heavy mediums solid constituent, has no deleterious effect on the heavy medium, and no effort need be made to remove such slime.

Based on the foregoing considerations, the present invention involves the removal (preferably by screening) from the underflow of the washing screens of substantially all mineral particles larger in size than the particles of the heavy mediums solid constituent, and treatment of the resultant underflow (preferably in a hydroseparator-thickener) to separate the heavy mediums solid constituent from substantially all other solid particles having slower free settling rates in water than the free settling rate in water of the particles of the heavy mediums solid constituent. The separated heavy mediums solid constituent is cleaned, thickened and returned to the heavy medium circulating through the separatory apparatus, advantageously by delivery to the aforementioned draining screens, the underflow of which is returned to the separatory apparatus along with the feed of mineral particles thereto. The slimes separated from the heavy mediums solid constituent may advantageously be utilized as wash water for the aforementioned washing screens, whereby slime particles adhere to the float and sink fractions passing across these screens and are removed with the fractions.

The foregoing and other novel features of the invention will be better understood from the following description taken in conjunction with the accompanying drawings, in which Fig. 1 is a diagrammatic flowsheet of an ore dressing plant embodying the invention, and

Fig. 2 is a diagrammatic flowsheet of an ore dressing plant embodying a modified form of the invention.

Preparatory to entering the ore dressing plant, the run-of-mine ore, coal or other mineral is crushed to a suitable and predetermined size, which in the case of bituminous coal may conveniently be to about 4 to 6 inches. The crushed mineral is fed onto a screen 5, where a portion of the fines are removed dry. The oversize of screen 5 passes to a screen 6 where a vigorous washing of the crushed mineral takes place. Used or dirty water may be used at the front or feed end of the screen 6, followed by fresh or clean water near the discharge end of the screen. Washing and screening should be as nearly complete and perfect as is possible to obtain with commercially available screening equipment and equipment and adequate wash water. A substantial portion of screen surface at the rear or discharge end of the screen 6 must be available, following the final addition of fresh water, to permit adequate drainage of surface wash water from the crushed mineral before it is discharged from the screen and fed to a separator 1 of the heavy media separation process.

The separator 1 may be of any conventional type, such as a drum, cone, trough or the like. In Fig. 1, a separator of the drum type is diagrammatically shown, in which the light or float fraction of the separation overflows a weir at one end of the drum and the heavy or sink fraction of the separation is elevated out of or otherwise appropriately separated from the heavy medium in the drum. The separated float and sink fractions, upon removal from the drum separator I, pass over drainage screens 8, where the greater portion of the heavy medium carried out of the separator with these fractions flows through the screen apertures and is re turned by a pump 9 to the separator. The float and sink fractions next pass to Washing screens Where wash water is sprayed or flowed over each fraction to remove heavy medium retained thereon.

For practice of the present invention the drainage and washing screens 8 and Ill have slightly smaller apertures than the apertures of the feed preparation screens and 6. The provision of this difference in screen apertures prevents contamination of the heavy medium (returned by pump 9 to the separator) with fine mineral particles contained in the feed to the separator, and facilitates recovery of the heavy mediums solid constituent from the underfiow of the washing screens, as more fully explained hereinafter.

A distinctive feature of the present invention is the washing on screens [0 of the float and sink fractions with water containing slimes. The slimes in this wash water are removed from the underflow of the screens H1 in the course of the treatment thereof for the recovery of heavy medium, as more fully explained hereinafter. The wash Water used on the screens Ill removes adhering heavy medium from the float and sink fractions, and the slimes in the wash water deposit upon and adhere to both fractions. The slime-coated float and sink fractions are discharged from the screens I0, and hence from the heavy medium circuit, and carry away with them such amount of mineral slime as coats or is attached to the solid particles of these fractions.

The underflow of the washing screens I0 contains the heavy medium and mineral slimes accompanying or associated with the fioat and sink fractions fed to the screens [0. This underflow is collected in a trough or chute beneath the screens [0 and directed to the head or feed end of a screen II. The apertures of the screen II are of such size as to remove solid particles larger than the maximum size particle of the heavy mediums solid constituent. A small amount of spray water is used on the screen ll to wash heavy medium off the relatively small amount of oversize mineral particles removed by this screen.

The underflow' of screen H is composed of water, particles of the heavy mediums solid constituent and mineral particles fine enough to pass through the apertures of the screen II'. This underflow is passed to a hydroseparator-thickener I2, whose function is to overflow water and low gravity slimes and to settle and thicken the dilute heavy medium fed to it. The overflow of the hydroseparator-thickener I2 is collected in a sump l3, and a variable amount of the overflow may be returned from the sump to the hydroseparator-thickener by means of a pump M and control valve l5. The valve ['5 is so adjusted that the rate of overflow return to the hydroseparator-thickener is such that substantially all of the low gravity mineral slime overflows and substantially all of the heavy medium solids settle and thicken. The flow rate between the sump l3 and the hydroseparator-thickener may vary from zero up to the maximum capacity of the pump 14, and the adjustment of the rate of return overflow will be determined by the size of the hydroseparator-thickener and the specific gravity and particle size of the mineral slimes to be removed in the overflow.

The sump I3 is connected through a pump I 6 with the spray nozzles or wash boxes I! above the Washing screens ill. The amount of liquid handled by the pump IE will depend upon the toniiage and size of mineral particles (float and sink fractions) being washed on the screens l0, and usually will vary from 1 to 3 gallons per minute per ton per hour of mineral particles being washed. Pumps I l and [6 may obviously be combined in a single pump, in which case the overflow rate of the hydroseparator-thickener [2 may be varied by either lay-passing a portion of the single pumps discharge to the feed of the hyd'rosep arator-thickener or by increasing or decreasing the amount of overflow delivered by the single pump as washer Water to the spray nozzles A pump [8 removes the clean and thickened heavy medium solids from the bottom of the hydroseparator-thickener l2, and delivers the thickened solids to a densifler or thickening device such as a spiral densifier E9. The heavy mediums solid constituent is further cleaned in the densifier l9, and is further thickened with respect to the dilution required in the operation of the pump la. The cleaned and thickened heavy medium solids from the densifier [9 are returned tothe heavy medium circuit of the separator "I along with fresh water supplied to the drainage screens 8. The light gravity slime with a small amount of heavy medium solids overflowing the densifier 19 are returned to the cleaning circuit with the feed to the hydroseparator-thickener [2.

The densifler or thickening device l9 serves both to thicken heavy medium from its pumpable density and to store medium. In normal operation, cleaned and thickened heavy medium is continuously returned from the densifier 19 to the heavy medium circuit of the separator I as rapidly as heavy medium (removed from that circuit) is washed off the mineral particles passing over the screens It. However, it is possible to raise the rakes in the spiral densifier I9 and store heavy medium therein, thus reducing the specific gravity of the heavy medium circulating through the separator 1. Conversely, it is possible to lower the spiral rakes in the densifier l9, and thus increase the amount of heavy medium solids in the circulatory system of the separator, thus raising the specific gravity of the heavy medium within the separator. The small amount of fresh water added to the solids discharge (thick medium) of the densifier I9 is controlled to bring the returned and thickened heavy medium to the operating density. In the case of a build-up of light gravity slime in the heavy medium circulating through the separator 1, beyond the normal cleaning herein described of the heavy medium underflow of the screens ID, a portion of the circulating heavy medium may be by-passed from the circuit of the separator 1 through a control valve 20 to the cleaning circuit.

That portion of the crushed mineral fines removed dry on the screen 5 passes to a belt conveyor 2|. fines removed by the wash water on screen 6 is collected in a dewatering spiral 22, and, after dewatering, joins the portion of fines removed dry on the conveyor 2 l. The overflow of the dewatering spiral 22 is conducted to a settling pond 23. A pump 24 lifts clarified water from the pond to the front or feed-end spray nozzles or wash boxes over the screen 6. The small amount of mineral fines removed on the screen I! are discharged onto the conveyor 2I. Any overflow of the sump I3 is piped to the settling pond.

The plant illustrated by the flow sheet of Fig. 2 is, in its main features, the same as the plant of Fig. 1, corresponding equipment being indicated by the same reference character in the two figures. In the plant of Fig. 2, the light gravity slimes are removed and disposed of in a separate circuit, instead of being carried away by the float and sink fractions as in the plane of Fig. 1. Thus, in Fig. 2, a pump 25 feeds overflow of the hydroseparator-thickener 12 from the sump I3 to a thickener 25. The thickened slimes are continuously removed from the thickener 26' by a diaphragm pump 21', and conveyed to a vacuum filter 28 for final dewatering and disposal. Water removed by the filter 28 is returned to the thickener 26 by means of a pump 29, and the filter cake (slimes) is discharged onto a conveyor 30, and thence deposited on the conveyor 2|. The overflow of the dewatering spiral 22 is conducted to the thickener 26. The overflow of the thickener 26 passes to a sump 3 I, and thence to the settling pond 23.

The following description of the treatment of bituminous coal and refuse illustrates, merely by way of example, a specific practice of the invention. The run-of-mine coal, crushed to 6 inches, is screened dry on the screen 5, wherein the screen apertures are such that the largest coal and refuse particles passing through the screen are inch in size (roughly 6 mesh Tyler screen). Similarly, the apertures of the wet screen 6 are such that the maximum size particle passing through the screen is A; inch. Thus, the screens 5 and 6 That portion of the crushed, mineral prepare and deliver to the separator a feed of coal and refuse particles ranging in size from 6 inches to /8 inch.

In the drum separator 1, coal particles, being of lighter specific gravity than the specific gravity of the heavy medium, float, while refuse particles, being of heavier specific gravity than the specific gravity of the heavy medium, sink. The resulting float and sink fractions of the separation are removed from the separator I and delivered to their respective drainage screens 8. In accordance with the invention, the apertures of the screens 8, in this example, are such that the largest size particle passing through each screen is not larger than inch (roughly 8 mesh Tyler screen). Similarly, the apertures of the screens iii are such that the maximum size particle passing through the screen is not larger than inch. By making the apertures of the drainage and washing screens 8 and it] slightly smaller than the apertures of the feed preparation screens 5 and 6, contamination of the heavy medium (returned to the separator T from the drainage screens 8) with naturally occurring fine coal and refuse particles is avoided. Hence, substantially the only coal or refuse particles entering the circulating heavy medium circuit are those produced by degradation of the coal and refuse while within the heavy medium circuit.

In this particular example, the settling rates in Water of particles of mixed coal and refuse of different sizes (20 mesh and smaller) are given in the following table. It will be noted that particles of these sizes pass freely through the screens 8 and ill.

Settling Rate Feet per Hour Also, in this example, the finely divided solid constituent of the heavy medium is magnetite ground to substantially all minus mesh, whose bulk settling rate in water is 33 feet per hour. It will thus be seen that the coal-refuse particles coarser than 100 mesh settle faster than the ground magnetite, and in the course of the operation of the heavy medium separation process would build up in the circulating heavy medium and so reduce its specific gravity that the process would become inoperative. According to the invention, the screen H removes all coarse coalrefuse particles whose settling rate in water is more rapid than that of the finely divided solid constituent of the heavy medium, and in this particular example the screen is 100 mesh, whereby all coal-refuse particles coarser than 100 mesh are screened out of the underflow of the washing screens It, washed free of any attached heavy medium, and removed from the cleaning circuit.

The mixture of heavy medium solids and mineral particles finer than 100 mesh enter the hydroseparator-thickener I2, where the overflow rate is maintained by the circulating pump it at a point just under the settling rate of the heavy mediums solid constituent (ground magnetite), which in this particular example is approximately 30 feet per hour. Under these conditions, coalrefuse particles substantially finer than 100 mesh accuser are overfiowed and substantially all particles of the heavy mediums. solid constituent. settle- In the practice of this particular example, I

have found when treating approximately 100 tons per hour of coal in the size range of 6 inches to A; inch, that approximately 600 pounds per hour of coal-refuse fines are produced within the heavy medium circuit, and appear in the underflow of the washing screens l. Of this 600 pounds per hour of fines, approximately 200: pounds. per hour are removed from the system on the screen ll, leaving 400 pounds per hour of slime (finer than 100 mesh) to be. removed by suitable means. I have further found that the float and sink fractions of the separation (in the size range of 6 inches to inch) have the capacity to collect slime. by adherence thereto to the extent of approximately 20 pounds of slimes per ton per hour of combined float and sink fractions. Since, in this particular example, about 100 tons of combined float and sink fractions are produced per hour, the slime-removal capacity of these fractions is about 2000 pounds perhour. As the amount of slime to be disposed of is only 400 pounds per hour, the ratio of slime-removal capacity to slime-production is to l, which is a wide margin of safety in. preventing a buildingup of slimes in the heavy medium circuit.

When treating minerals other than bituminous coal, and when using a material other than magnetite as the solid constituent of the heavy medium, the relation between the size of the apertures of the screen ii and the overflow rate of the hydroseparator-thickener It will vary depending upon the dilierent factors entering into the specific gravity size relationship. Generally speaking, screen i i removes substantially all mineral particles having a more rapid free settling rate in water than that of the maximum size particles of the heavy mediums solid constituent, and the overflow rate of the hydroseparatorthickener i2 is so adjusted that substantial-1y all of the remaining finer mineral particles overflow While substantially all particles of the heavy mediums solid constituent sink.

The invention provides a simple and inexpensive method of maintaining the heavy medium relatively free from contaminating slimes, and of recovering and cleaning heavy medium with the separation therefrom and disposal of slimes:

produced in the normal functioning of the heavy medium separation process. The invention permits the use of the most available raw material locally obtainable in the neighborhood of the ore dressing plant as the solid constituent of the heavy medium. Thus, practice of the invention permits the use as the heavy mediums solid constituent of such cheap and readily available materials as flue dust from iron blast furnaces, iron ore sintering plants and the like, hematite, limonite, magnetite, barytes, galena, sphaelerite, ilmenite, etc.

I claim:

1. In the concentration of mineral particles of diiierent specific gravities by the heavy media separation process in which the mineral particles are subjected to wet screening preparatory to the separation step and the float and sink fractions of the separation step are respectively drained and washed on a sequence of screens, the improvement in recovering the finely divided solid constituent of the heavy medium accompanying the float and sink fractions delivered to their respective washing screens which comprises. draining. and washing the float and sink fractions.

on screens ofsubstantially smaller aperture size than the aperture size of the wet preparatory screen, delivering the under-flow of said washing screens toanother screen on which the solid particles removed asoversize have a free settling rate inwater at least as rapid as the free settling rate of the maximum size particles of the finely divided solid constituent of the heavy medium, delivering the underfiow of said other screen to a hydrosepa-rator-thickener, controlling the overflow of said hydroseparator-thickener by the return thereto of some of said overflow to remove in" the overflow substantially all solid particles having a slower free settling rate in water than the free. settling rate of the finely divided solid constituent of the heavy medium, and recovering from the thickened discharge of said hydroseparator-thickener the finely divided solid constituent of the heavy medium contained therein.

2.. Inthe concentration. of mineral particles of different specific gravities by the heavy media separation process in which the mineral particles are subjected to wet screening preparatory to the separation step and; the float and sink fractions of the separation step are respectively drained and washed on a sequence of screens, the improvement which. comprises draining and washing. the floatv and sink fractions on screens of substantially smaller aperture size than the aperture size of the wet preparatory screen, delivering. the underflow of said washing screens to another screen. on which substantially all solid particles larger in size than the particles of the solid constituent of the heavy medium are removed in the oversize, delivering the underflow of said last-mentioned other screen containing no solid particles. of larger. size than the heavy mediums solid constituentto. a hydroseparatorthickener, controlling. the overflow of said bydroseparator-thiekener by the. return thereto of some of said overflow toremove in the. overflow substantially all solid. particles having a slower free settling rate. in. water than the free settling rate. inwater of. the particles of the solid constituent of the heavy medium, recovering from the thickened discharge of said hydroseparatorthickener the particles. of the solid constituent of the heavy medium contained therein, and delivering overflow from said hydroseparatorthickener to: saidwashing screens as wash water for the float and sink: fractions whereby slimes in1said; overflow adhere. to said fractions and are removed therewith.

References. Citedin the flle of this patent UNITED STATES PATENTS Number. Name Date 2,139,789 Wuensch Dec. 13, 1938 2,325,149 Rakowsky et al July 2'7, 194:3

FOREIGN PATENTS Number Country Date 616,425 Great Britain Jan. 20, 19%9 617,011 Great Britain Jan. 31, 1949 855,377 France May 9, 1940 OTHER REFERENCES Quarterly of the Colorado School of Mines, January 1948, vol. 43, No. 1 pages 46 and 47. 

1. IN THE CONCENTRATION OF MINERAL PARTICLES OF DIFFERENCE SPECIFIC GRAVITIES BY THE HEAVY MEDIA SEPARATION PROCESS IN WHICH THE MINERAL PARTICLES ARE SUBJECTED TO WET SCREENING PREPARATORY TO THE SEPARATION STEP AND THE FLOAT AND SINK FRACTIONS OF THE SEPARATION STEP ARE RESPECTIVELY DRAINED AND WASHED ON A SEQUENCE OF SCREENS, THE IMPROVEMENT IN RECOVERING THE FINELY DIVIDED SOLID CONSTITUENT OF THE HEAVY MEDIUM ACCOMPANYING THE FLOAT AND SINK FRACTIONS DELIVERED TO THEIR 